Portable systems, such as mobile electronic devices, which are powered by rechargeable batteries have a problem supporting both USB (Universal Serial Bus) charging and suspend functions.
In operation, USB specifications require that any devices which are connected to a USB host initiate enumeration within ˜150 msec of a USB cable being attached, hereon referred to as “VBUS detection”. Enumeration is the process whereby devices attached to the USB host request permission to access the host. In the present invention, the enumeration request is directed to a request to draw power from the USB host in order to power up the mobile electronic device which has a dead or non-present battery.
When the rechargeable battery is dead or not present, the mobile electronic device can not operate since it does not have any power. In most cases, it is desired that a battery charger within the mobile electronic device turn on once it receives power from the USB VBUS power line upon VBUS detection. This causes the charger to be enabled so that power is supplied from the USB host for operation of the device and recharging of the battery. This may be referred to as a device charging state. Therefore, when the voltage via the VBUS is applied, the charger turns on and acts as the battery to power the CPU along with charging the battery. In this case, all the signals to the battery charger are in a low state.
Another common state for the mobile electronic device is a device suspend state. USB specifications require that a total USB supply current to the mobile electronic device not exceed 500 μA in the device suspend state. With many mobile electronic devices, 500 μA is not enough current for the CPU of the mobile electronic device to operate and therefore the device should be powered down. Powering down of the CPU causes all the control signals to default to a low logic level state, which would then keep the charger on. This state of the charger is not desirable for the system, during a device suspend state. In some prior art devices, two separate signals to control the device charging state and the device suspend state are used.
In some other prior art devices, support for the device suspend state is not recognized and the battery charger remains enabled during the device suspend state. In this manner, the 500 μA current limit is not recognized by the mobile electronic device even though it is required under the USB specifications.
Therefore, there is provided a method and apparatus for handling a charging state in a mobile electronic device.